The titles of axes in a plot usually consists of units of some physical parameter. How do we represent such a unit, especially when it contains some letter raised to the power some number? For example, if the x-axis represents velocity, how can we represent m s^-1 properly as the xtitle?
It depends on the graphics system you are using.
If you are using function graphics, then you can do something like:
IDL> p = plot(/test, ytitle='velocity $m s^{-1}$')
If you are using direct graphics, then do:
IDL> plot, findgen(10), ytitle='velocity m s!E-1!N'
See the Doc Center for a complete listing of the embedded graphics format codes.
There are also helpful library routines to help with direct graphics.